1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine and a laser beam printer, which allows an electrostatic latent image on a latent image bearing member to be a visible image by a development device.
2. Description of Related Art
As illustrated in FIGS. 7 and 8, an image forming apparatus typically has a photosensitive drum 1 and a development device 3. The development device 3 has a developing sleeve 12. The photosensitive drum 1 and the developing sleeve 12 are arranged so as to constantly hold an SD gap Gsd which is a small gap between the photosensitive drum 1 and the developing sleeve 12, thereby performing predetermined development. When the SD gap Gsd is varied, poor image formation such as inconsistencies in density in an image can be caused.
The developing sleeve 12 has an abutment roller 20 at either end in a longitudinal direction so as to be rotatably held via a bearing 24, and is rotatably supported by a developing case 22 via a bearing 21. The developing sleeve 12 has at one end a gear 23 which provides the driving force of rotation to the developing sleeve 12. The driving force is inputted from the outside of the development device 3. The SD gap Gsd is ensured by the abutment roller 20 which is provided at either end of the developing sleeve 12 and a to-be-abutted portion.
The abutment roller 20 abuts on the outer circumferential surface of the photosensitive drum 1 as the to-be-abutted portion. The developing sleeve 12 is arranged so as to hold the predetermined SD gap Gsd.
The abutment roller 20 needs to be rotated following the photosensitive drum so as to prevent frictional sliding of the surface of the photosensitive drum 1 and the abutment roller 20. The abutment roller 20 is thus rotatably held by the developing sleeve 12. The development device is pressed to indirectly press the abutment roller 20 onto the photosensitive drum 1. To abut the abutment roller 20 on the circumferential surface of the photosensitive drum 1, the base end of a spring 18 is attached to the rear side of the developing case 22. The end of the spring 18 is supported by a fixing plate 19 which is provided in the image forming apparatus body. The development device 3 is pressed toward the photosensitive drum 1 by a horizontal force.
The abutment roller 20 is provided via the bearing 24 so as to be rotated with the photosensitive drum 1 on which the abutment roller 20 abuts. The inner circumference of the bearing 24 is rotated with the developing sleeve 12.
The development device 3 can be detached by removing the fixing plate 19 so as to be slid along guide members 13 and 14 when the used developing sleeve 12 needs to be maintained.
As illustrated in FIGS. 9 and 10, the abutment roller 20 abuts on an abutment block 16 which has a high-precision surface S provided so as to be concentric with the photosensitive drum 1. The developing sleeve 12 is arranged so as to hold the predetermined SD gap Gsd.
The development device 3 is vibrated when the driving force is inputted from the outside. Variation in the SD gap Gsd caused by the vibration can be reduced by increasing the spring force of the spring 18 (see Japanese Patent Application Laid-Open No. 11-15269).
When the spring force is increased in the image forming apparatus of the related art, the radial load of the bearing 21 which receives the developing sleeve 12 and the radial load of the bearing 24 of the abutment roller 20 are increased, thereby deteriorating the durability of both the bearings.
As illustrated in FIG. 11, when the abutment force of the abutment roller 20 on the abutment block 16 is F [N], the force of the spring 18 also needs F [N]. When the bearing 21 is integrally held by the developing case 22, the bearing 21 is sandwiched between the spring force and the developing sleeve 12 to receive the radial load of F1 [N] (F=F1).
When the bearing 24 is integral with the abutment roller 20, the bearing 24 is sandwiched between the abutment block 16 and the developing sleeve 12 to receive the radial load of F2 [N] (F=F2).
The increased radial load leads to increase in the torque of the developing sleeve 12. The vibration of the development device 3 at transmission of the driving force can deteriorate the durability of the gears of the driving train. The increased torque of the developing sleeve 12 can increase rotational variation of the developing sleeve 12 due to engagement per tooth of the developing driving train. Inconsistencies in pitch appear on an image.